Penning type vacuum pumps



Nov. 14, 1967- L. A. HOLLAND 3,353,054

PENNING TYPE VACUUM PUMPS Filed June 9, 1964 3 Sheets-Sheet 1 H646 I .vI

IHVEMTQE L E A.HOLLAND ATTOENEY- Nov. 14, 1967 L. A. HOLLAND PENNINGTYPE VACUUM PUMPS 3 Sheets-Sheet 5 Filed June 9, 1964 55 950 z Tmobmmawwwma 35% ig 200% o om I O Y N m [935/1] oaads smaw'nd Imvanrroe L AHon/VD BY Mf- ATTORNEY Patented Nov. 14, 1967 3,353,054 PENNING TYPEVACUUM PUMPS Leslie Arthur Holland, Crawley, England, assignor toEdwards High Vacuum International Limited, Crawley, England, a Britishcompany Filed June 9, 1964, Ser. No. 373,798

ABSTRACT OF THE DFSCLQSURE A getter ion pump of the Penning type whichinduces spiralling of electrons between two parallel cathodes through agrid-like anode in which each cathode has raised and depressed portionsjoined by side walls which are available for redeposition of materialand gas removed during sputtering of the cathode. This results inimproved stability and increased pumping rates when pumping eitheractive or inert gases.

' follow spiral paths thereby increasing their ionization efficiency.The cathodes are made of chemically active metal, for example titanium,zirconium, etc. Positive ions attracted to the cathodes sputter metalatoms which then deposit onto the anode and envelope wall where theyabsorb gases.

It is known to construct a pump of this nature provided with a number ofindividual Penning pumps in parallel enclosed within a single evacuableenvelope using a cellular anode of box-elements between two cathodes oflarge area. Ion bombardment of the cathodes sputters active metal fromthe cathode surfaces which is then deposited on the surfaces of themulti-cellular anode.

A fundamental problem in such pumps is the etficient sorption of bothactive and inert gases. When the pump is operative to exhaust gas with acomposition of air, for example, then the cathodes are bombarded by bothactive and inert gas ions which partly penetrate the cathodes saturatingtheir outer surfaces. As ion bombardment continues the gases beingion-pumped are released by the sputtering of the cathode metal andpumping will cease when the rate at which gases are released from thecathodes equals the rate at which they impinge upon the cathodes.Neutral atoms and molecules of certain active gases are still removed bygettering at the anode surfaces but the inert gases cannot be sorbed atthe anode by chemical reactions; even though they may be renderedtemporarily active by the electrical discharge this does not re- I sultin any measurable sorption rate. Furthermore metal getters do notcombine with equal readiness with all active gases and with a mixture ofgases one specie may be chemically sorbed at the expense of another.

Thus a normal Penning pump as described above with either a unit ormulti-cellular anode will, after a period, exhaust only active gases,and at a rate depending upon the gas composition and the rate at whichsputtered material creates a fresh gettering surface on the anode.

The purpose of this invention is to provide a region of sputteredmaterial which can sorb non-ionised active gases and ions of both activeand inert gases.

According to the present invention an electrical vacuum getter pumpcomprises an evacuable envelope containing at least two cathodes, ananode of grid construction, and means providing a magnetic fieldtransverse to the planes of the cathodes, the two cathodes having asurface made up of an alternate array of raised and depressed polygonalsections joined by side walls disposed to receive sputtered materialfrom said depressed section in operation of the pump, the plane in whichthe raised sections and the plane in which the depressed sections liebeing in spaced parallel relation. The area of each raised and depressedportion and the area of each anode grid aperture are approximatelyequal, the distance between the depressed sections and the raisedsections of each cathode measured in a direction at right angles to thecathode planes is not larger than the distance between adjacent raisedsections, and the side walls of each of said cathodes are disposedsubstantially opposite the grid outline of the anode. 1

In one embodiment of the invention both cathodes have surfaces made upof alternate arrays of raised and depressed rectangular sections, theraised portions of one cathode being mounted opposite the depressedportions of the other cathode, each cathode having at least one raisedand one depressed rectangular section.

In another embodiment the raised portions of both cathodes are mountedopposite each other.

In any of the constructions embodying the invention some or all of theside portions joining the raised and depressed rectangular sections ineither or both cathodes may be mounted inclined to a plane perpendicularto the planes of the rectangular sections by an angle between 0 and 80.

Preferably the rectangular sections in any embodiments of the inventionare square.

In a Penning discharge using plane cathodes with a hollow anodeconstruction the central region of the cathode is exposed to the mostintense bombardment by positive ions. Consequently, most of thesputtered metal is liberated from the central region of the cathode.Thus in the several embodiments of the present invention the sputteredmetal emitted from the intensely bombarded region, namely the depressedsections, is deposited on the sides'of the open cathode boxes. A weakcurrent of positive ions flows to the sides of the boxes, but thesputtering it produces is small compared to that of the intenselybombarded region. Consequently there isa net transfer of cathodematerial onto the sides of the boxes. The sputtered metal is then placedin a region where it can sorb neutral atoms and molecules of activegases,

and a proportion of the total ions of both active and inert gases.Employing a large number of cathode cells enhances the pumping speed.

The invention will now be described in greater detail with reference tothe accompanying drawings in which:

FIGURES 1a and. 1b each show a schematic cross-section of unit cells;

FIGURES 2a and 2b each show a schematic cross-section of a multi-cellconstruction;

FIGURE 3 shows a perspective view of the cathodes and the anode;

FIGURES 4a and 4b each shows diagrammatically further sections ofmulti-cell constructions;

FIGURE 5 is an elevation, partly in section of a pump embodying theinvention; and

FIGURE 6 shows curves of pumping speeds.

Referring firstly to FIGURE 3 of the drawings an anode 1, is of gridform constructed in two sets of equally spaced parallel conductivewires, the two sets being at right angles positioned in a plane parallelto and midway between planes containing the cathodes 2 and 3. Bothcathodes comprise a square array of alternately open and closed boxes.The numeral 4 denotes one such open box in the cathode 3 and 5 denotes aclosed box. Each of cathodes 2 and 3 comprises an array of 24 unitcells, 12 closed and 12 open. The two cathodes are mounted with the openboxes in each cathode opposite the open boxes in the other cathode. Forexample open box 4 in cathode 3 is opposite open box 6 in cathode 2 andclosed boxes 5 and 7 are likewise disposed. FIGURE 1a shows a verticalsection through a unit cell, the two open box cathode portions disposedeither side of the grid anode 1. The direction of the applied magneticfield is shown by the, letter H and associated arrows. FIGURE 2a shows avertical section through a multiple array of unit cells as illustratedin FIGURE 3 with cathode portions disposed on either side of the anodegrid 1.

In operation of the pump the cathodes 2 and 3 are held at the samenegative potential with respect to the anode potential. A magnetic fieldis applied in the direction described. For operating the pump a typicalcombination of applied Voltage and magnetic field would be 6 kv. at1,000 gauss. However, the apparatus is not limited to this operatingcondition, which depends on the cathode/anode dimensions and spacing.Normally the pump will not be operated until a pressure of about 0.1 mm.Hg has been obtained by conventional vacuum techniques. Electrons passfrom cathodes to the anode executing spiral paths and ionising gas atomsand molecules in their path. The positive ions thus provided areattracted to the cathode and impinge thereon. The active gases presentcombine chemically with the cathode whilst ions of inert or chemicallyactive gases partly penetrate the surface of the cathode. Metals atomssputtered from the cathode surfaces are dislodged preferentially fromthe centre of the square sections and gas particles trapped in or on thesurface of the cathode may become dislodged at the same time. Sincehowever the discharge current of positive ions is greatest at the centreof the square sections there is a net transfer of metal atoms onto theside walls of the boxes. For example sputtered metal from the base 8 ofbox 4 is deposited onto the walls 9, and the two walls adjacent 9 and10. These surfaces therefore provide areas in which gaseous ions andneutral atoms or molecules may become trapped by sputtered metal atomstransferred from the base 8. Neutral atoms or molecules may also begettered by sputtered metal deposited on the walls of the envelope ofthe pump, not shown, and the anode.

The pump described above is only one embodiment of the invention and itis possible to employ alternative arrangement of the cathodes, or evenalternative constructions of the anode.

For example FIGURE 1b and FIGURE 2b show respectively sections of a unitcell and a multicell in which the cathodes are arranged with the raisedportions 11 0pposite depressed portions 12.

Alternative arrangements in the construction of the unit cell areillustrated in FIGURE 4a and FIGURE 412. FIGURE 4a illustrates a sectionof a pair of cathodes 13 and 14 the raised and depressed sections ofwhich, 11, 12 respectively, are joined by side walls 15 inclined to theplane of the raised or depressed portions by an angle greater thanFIGURE 2a represents a section along theline AA in FIGURE 3. It is to beunderstood thatthe walls parallel to wall 9 in FIGURE 3 are at 90 to theplane of the raised or depressed portions.

FIGURE 4b illustrates a similar section in which the cathode boxes arere-entrant the angle 0 being less than 90, the angle in FIGURE 3 being90.

Referring now to FIGURE 5, the pump shown consists of a body 16supporting a magnet 17 and provided with a terminal 18 for the operatingvoltage supply leads for the electrodes. The anode 1 is of the formshown in FIGURE 3 and the cathodes 2 and 3, composed of titanium are ofthe general form also shown in FIGURE 3. The cells of the cathodes arehalf an inch square and a quarter of an inch deep, the opposed closedfaces of:

the cells being spaced one inch apart. The anode is disposed midwaybetween the cathodes and is composed of wire inch diameter. The magnet17 provides a field of 1,000 gauss and the operating voltage is 6 kv.

The pumping speeds indicated by the curves shown in FIGURE 6 are inlitres per second at a specific pressure and the pressures in torrvalues. Thus, in the particular example of the pump being described, thepumping speed for air as shown by curve 19 is 38 hIres per second at apressure of 10" torr measured at 10- torr. The .volumetric speeds may bemeasured using two chambers one of which is constituted by the pump, thetwo chambers being separated by an orificed plate. The pumping speedsare determined by measuring the pressure in the two vessels, theconductance of the orifice being known.

Curve 20 shows the pumping speed obtained for argon in the case of thepump described. Thus, the pumping speed for argon may be expected to beof the order of 14 litres per second at a pressure of 10- torr measuredat 10- torr.

It will be understood that the invention may be carried out in waysdifferent from the particular embodiment described. For example, thedimensions of the electrodes, their relative spacing and the operatingvoltages may be selected to suit required conditions.

I claim:

1. An improved electrical vacuum getter pump of the type provided with(A) an electrode assembly comprising (1) two substantially parallelspaced cathodes and (2) an anode of grid construction disposed betweensaid cathodes, and

(B) means for impressing a magnetic field across said electrode assemblyin a direction substantially normal to the cathode planes to causeelectrons moving in the space within said assembly to follow spiralpaths between the said cathodes before being captured, the improvementresiding in the factthat (C) each of said cathodes has a surface made upof an alternate array of raised and depressed polygonal sections joinedby side walls disposed to receive sputtered material from said depressedsection, the area of each raised and depressed section and the area ofeach grid aperture of said anode are approximately equal, the distancebetween said depressed sections and said raised sections measured in adirection at right angles to the cathode planes is not larger than thedistance between adjacent raised sections, and said side walls of eachof said cathodes are disposed substantially oppositethe grid outline ofsaid anode.

2. An improved pump according to claim 1 wherein said raised anddepressed sections are rectangular, the raised portions of one cathodebeing disposed opposite the depressed portions of the other cathodes.

3. An improved pump according to claim 1 wherein said raised portions ofone cathode are disposed opposite said raised portions of the othercathode.

4. An improved pump according to claim 1 wherein said side walls areinclined to a plane perpendicular to the cathode planes by an anglebetween 0 and 80.

5. An improved pump according to claim 1 in which said distance betweensaid depressed sections and said raised section is one half of thedistance between adjacent raised sections.

5 References Cited UNITED STATES PATENTS 3,070,719 12/1962 Jepsen 230-69X 3,228,589 1/1966 Kearns 3137 X 10 JAMES W. LAWRENCE, Primary Examiner.S. A. SCHNEEBERGER, Assistant Examiner.

1. AN IMPROVED ELECTRICAL VACUUM GETTER PUMP OF THE TYPE PROVIDED WITH(A) AN ELECTRODE ASSEMBLY COMPRISING (1) TWO SUBSTANTIALLY PARALLELSPACED CATHODES AND (2) AN ANODE OF GRID CONSTRUCTION DISPOSED BETWEENSAID CATHODES, AND (B) MEANS FOR IMPRESSING A MAGNETIC FIELD ACROSS SAIDELECTRODE ASSEMBLY IN A DIRECTION SUBSTANTIALLY NORMAL TO THE CATHODEPLANES TO CAUSE ELECTRONS MOVING IN THE SPACE WITHIN SAID ASSEMBLYFOLLOW SPIRAL PATHS BETWEEN THE SAID CATHODES BEFORE BEING CAPTURED, THEIMPROVEMENT RESIDING IN THE FACT THAT (C) EACH OF SAID CATHODES HAS ASURFACE MADE UP OF AN ALTERNATE ARRAY OF RAISED AND DEPRESSED POLYGONALSECTIONS JOINED BY SAID WALLS DISPOSED TO RECEIVE SPUTTERED MATERIALFROM SAID DEPRESSED SECTION, THE AREA OF EACH RAISED AND DEPRESSEDSECTION AND THE AREA OF EACH GRID APERTURE OF SAID ANODE AREAPPROXIAMTELY EQUAL, THE DISTANCE BETWEEN SAID DEPRESSED SECTIONS ANDSAID RAISED SECTIONS MEASURED IN A DIRECTION AT RIGHT ANGLES TO THECATHODE PLANES IS NOT LARGER THAN THE DISTANCE BETWEEN ADJACENT RAISEDSECTIONS, AND SAID SIDE WALLS OF EACH SAID CATHODES ARE DISPOSEDSUBSTANTIALLY OPPOSITE THE GRID OUTLINE OF SAID ANODE.